If you're still using the Julian Calendar (and I hear that all the Cool Kids are), then this is the last day of the year. Don't over do it with the noise makers and Champagne, you whacky hipsters!
Of course, the Julian calendar is named for Julius Caesar, because he had the Roman, err, calendar makers create it. Back in around 45 B.C. Since it was off by a quarter day a year, by 1500 A.D. or so, things were really out of whack. Like Mid-winter coming in spring out of whack.
Pope Gregory had the, err, roman calendar makers come up with a new one - the Gregorian calendar. With an extra quarter day a year - a leap year every 4 years. They also put spring back where it belongs (in spring). So New Years was moved back to, well, New Years, where it remains to this day.
Some old conservative fuddy duddys (probably Republicans or something) didn't like this at all. They kept celebrating the New Year with a week's worth of part-ay at the end of March. Still hung over on April 1, they were the "April Fools".